It says +45% bounce armor pen. How dose that work? Do I need to get a ghost blade first and then it only gives me 9 more to the GBs 20? It would not stack with Black Cleaver then either. So it only works with 
and 
?
Zielmann2 votesIt means it penetrates 45% of the target's bonus armor, not that it increases flat armor pen by 45%
KyroRayne2 votesLast whisper needs the enemy champions to have Bonus armor for the Armor Pen to work. If they have no bonus armor, it's useless against them.... and they're not that bright for not having armor seals or masteries, but that's another story :P
CitadelOfIce1 votesLast whisper needs the enemy champions to have Bonus armor for the Armor Pen to work. If they have no bonus armor, it's useless against them.... and they're not that bright for not having armor seals or masteries, but that's another story :P
Your words made me chuckle because I think I'm the only ADC/JG/SUP/MID that goes out of there way to build armor or MR in my games.
Raka Flaka MD1 votesNo you do not need armor pen already for it to work.
MistroTTC1 votesHere is how it works, say a champ has 50 armor without items, but bought a thornmail, the thornmail grants 100 bonus armor, so 45 percent bonus armor pen will ignore 45 armor so it acts like you only have 115 armor. The armor without the items is not affected by bonus penetration, though it is penetrated by flat pen and the armor penetration runes.
Bonipherus1 votes"Bonus" armor refers to armor a champion doesn't naturally have. So if someone is building 
and 
it's like you pierce 45% of the armor gained by those items. This means you are only dealing with 60.5 additional armor that champion has in comparison to the 110 additional armor they actually built.
Vistha Kai1 votesIt's 45% of target's bonus armor.
So, say, the target has 50 base(without items, masteries, etc.) and buys a 100 armor. 45% bonus armor penetration will make you ignore 45 points of target's bonus armor (45% of 100).
The result is you'd deal damage as if they had 105 armor (50 base + 55 remaining from the item).
EndlessSorcerer1 votesIt only penetrates bonus armor.